Sequencer

Sequencer

Hand out identifiers that are unique across every node without a central bottleneck — and decide whether those IDs must also be sortable or monotonic. Getting this wrong shows up late and hard: collisions corrupt data, a single allocator caps write throughput, and IDs that leak a creation time or a sequential count expose business secrets and enable enumeration attacks.

When to reach for this

A system writes new records across multiple nodes and each needs a primary key (orders, messages, uploads, events). Reach for this when a single auto-increment column would serialize all writes, when IDs must be generated before a DB round trip (client-side, offline), or when records must be roughly time-ordered without a separate sort field.

When NOT to

A single relational node still comfortably serves the write load (→ back-of-the-envelope) — then a plain BIGINT AUTO_INCREMENT/SERIAL is the cheapest correct answer; do not build a distributed ID service for it (YAGNI). If a natural unique key already exists (email, ISBN, content hash), use it. Don't demand global monotonicity unless an invariant truly needs it — it is the most expensive property here and usually only per-entity ordering is required.

Clarify first

• Generation point — client/edge, app server, or database? (Decides whether a DB round trip per ID is acceptable.)
• Ordering need — none, time-sortable (k-sorted is fine), or strictly monotonic? Per-entity or global? This is the single biggest fork.
• Write rate & node count — IDs/sec at peak and how many generators (→ back-of-the-envelope). Sets the bits needed for a sequence counter.
• Size & encoding budget — 64-bit int (fits an indexed key cheaply) vs 128-bit (no coordination ever) vs short URL-safe string?
• Leakage tolerance — may the ID reveal creation time or a guessable count (enumeration / competitor signal)?

The options

• Auto-increment / SQL sequence — one DB column hands out IDs. Use when a single node owns the writes and you want zero new infrastructure.
• UUIDv4 (random 128-bit) — generate anywhere, no coordination, effectively zero collision risk. Use when you only need uniqueness and never sort by ID.
• ULID / UUIDv7 (time-prefixed 128-bit) — random but with a millisecond timestamp prefix, so IDs sort by creation time. Use when you want UUIDv4's zero-coordination and time-ordering (the modern default for new keys).
• Snowflake-style (timestamp + node + sequence, 64-bit) — pack a timestamp, a node ID, and a per-ms counter into a sortable 64-bit int. Use at high write rates where a compact, k-sorted integer key matters.
• DB ticket / range allocation (Flickr-style) — a central table hands out blocks of IDs (e.g. 1000 at a time); each node serves from its block in memory. Use when you want simple monotonic-ish integers without per-ID coordination.

Trade-offs

Behavior under stress

The whole point of distributed ID schemes is to avoid a single allocator, so the failure modes cluster around coordination shortcuts.

• Allocator as SPOF/bottleneck (ticket, single sequence): every write blocks on one row/node. A spike or its failure stalls all inserts. Mitigate: hand out larger ranges, replicate the allocator, or move to Snowflake/ULID (no central hop). Larger ranges trade away monotonicity and waste IDs on restart.
• Clock skew & rewind (Snowflake/time-prefixed): if a node's clock jumps backward (NTP correction, VM pause), it can re-emit a timestamp it already used and collide within its node+sequence space. Mitigate: refuse to emit while now < last_timestamp (block or error), use a monotonic clock source, and alarm on skew. Never silently trust wall-clock time.
• Sequence-bits exhaustion: more than 2^seq_bits IDs in one millisecond on one node overflows the counter. Mitigate: spin-wait to the next ms, or size the bit budget to peak rate up front.
• Node-ID collision: two generators boot with the same node ID (bad config, autoscaling reuse) and silently mint duplicates. Mitigate: lease node IDs from a coordinator (→ consistency-coordination) instead of static config.
• Hot shard from sequential keys: monotonic IDs as a shard/partition key send all new writes to one shard. Mitigate: hash the key or prefix-shard — the partitioning fix lives in data-storage.

Monitor: ID issuance rate per node, clock-skew/rewind events, allocator latency and range-exhaustion rate, and duplicate-key errors (should be zero).

How to apply

1. Clarify the inputs — pin down the generation point, the ordering need (none / time-sortable / strict; per-entity vs global), peak IDs/sec, the size budget, and leakage tolerance (see Clarify first). If one DB node serves the writes, stop — use auto-increment (→ back-of-the-envelope).
2. Pick from the trade-off table — no ordering and 128-bit is fine → UUIDv4; want time-sortable with zero coordination → ULID/UUIDv7; need a compact 64-bit k-sorted int at high rate → Snowflake; want simple monotonic ints → ticket/range.
3. Set the key knobs — for Snowflake fix the epoch and the timestamp/node/seq bit split against your node count and peak rate; for ticket pick the block size; decide the node-ID assignment method (lease vs static); choose the encoding (raw int, Base62, Crockford Base32).
4. Stress-test the choice — walk Behavior under stress: clock rewind, sequence exhaustion, node-ID collision, allocator failure, and the hot-shard effect of sequential keys. Confirm a mitigation for each one your profile hits.
5. Size it with numbers — confirm the bit budget covers peak IDs/sec/node and the epoch gives enough years; confirm the encoded length fits the key/URL constraint (→ Numbers that matter).
6. Pick a provider — default to the generic recipe (a Snowflake library, a ticket row, or ULID/UUIDv7); only open a provider file if the user named a cloud (see Choosing a provider).

Dos and don'ts

Do

• Default to ULID/UUIDv7 for new keys when you want zero coordination plus rough time-ordering — it dodges UUIDv4's random-index pain.
• Size the Snowflake bit budget (timestamp/node/sequence) against peak rate and required lifespan before picking it; write the epoch down.
• Lease node IDs from a coordinator instead of static config when nodes autoscale.
• Refuse to emit on clock rewind and alarm on skew; treat duplicate-key errors as a P1.
• Separate the internal key (sortable, may leak time) from any external opaque ID when enumeration or leakage matters.

Don't

• Don't build a distributed ID service before a number shows one DB node can't keep up (YAGNI).
• Don't use a random UUIDv4 as a clustered/primary index on a hot table — random order causes page splits and write amplification.
• Don't make a single sequence or ticket row the allocator for the whole fleet without replication — it's a SPOF and a write bottleneck.
• Don't use a globally monotonic ID as a shard key — it creates a hot shard (fix in data-storage).
• Don't trust wall-clock time for ordering; ms-sortable is k-sorted, not strict.

Numbers that matter

A 64-bit Snowflake layout (≈41 timestamp bits + 10 node + 12 sequence) gives ~69 years from its epoch, 1024 nodes, and 4096 IDs/node/ms ≈ 4M IDs/node/sec — ample for almost any single service. UUID/ULID are 128 bits = 16 bytes (vs 8 for a 64-bit int), doubling index key size. A ticket block of N IDs cuts allocator hits by N× but risks losing up to N IDs on a node restart. For peak-rate and storage sizing, see back-of-the-envelope; restate only the figure a decision turns on.

Interface sketch

An issued ID is a contract. State its width (64 vs 128 bit), its layout (e.g. Snowflake [timestamp:41 | node:10 | seq:12]), its ordering guarantee (unordered / k-sorted by ms / strict), and its encoding (raw int, Base62, Crockford Base32 — case-insensitive, URL-safe). A generator endpoint, if any, is minimal: next(entity) -> {id, issued_at}. Document whether the ID is the storage key, the sort key, or both — that choice is consumed by data-storage.

Choosing a provider

Default to the generic recipe above. If the user names a cloud, read references/providers/<provider>.md for the managed-service mapping, quotas/limits, and provider-specific trade-offs. If no file exists for that provider, the generic recipe is the answer (most clouds have no dedicated ID service — you run a library or a sequence yourself).

Diagram

To visualize the issuance path (generator nodes → 64-bit layout → record key) or the ticket-server block-allocation flow, use the in-plugin architecture-diagram skill; an inline [ts | node | seq] sketch is enough for quick bit-budget reasoning. Do not embed Mermaid.

Related building blocks

• data-storage — feeds into it: the ID becomes the primary/sort key, and the sharding/partitioning that a sequential key can hot-spot is owned there.
• consistency-coordination — depends on it for the causality, ordering, and leader-election theory behind monotonic guarantees and node-ID leasing; link, don't re-teach.
• messaging-streaming — pairs with it for message ordering and dedup, where time-sortable IDs give a natural sequence and idempotency anchor.
• api-design — pairs with it: ID generation underpins idempotency keys (owned there) for safe retries.
• system-design — owned-concept lives in the orchestrator: the reasoning loop, the trade-off method, and the ten failure modes.

References

• references/deep-dive.md — Snowflake bit-layout math and epoch choice, clock-skew/monotonic-clock handling, ticket-server range allocation, ULID vs UUIDv7 byte layout, encoding (Base62/Crockford), and node-ID leasing. Read when designing the generator in detail.
• references/providers/{generic,aws,gcp}.md — library/service mappings, limits, and pitfalls per environment.